In a wireless Local Area Network (LAN) which is configured with only the terminals that does not require an access point and capable of establishing wireless connection, the network configuration can be dynamically modified. Such a network is generally referred to as an ad-hoc network or a mesh network.
Terminals that can establish wireless connection in the network include, for example, personal computers, Persona Digital Assistants (PDA), mobile phones, and terminals for on-vehicle car navigation system.
FIGS. 15A and 15B are diagrams showing examples of networks having a wireless network configuration which can be dynamically modified.
Such a network includes, as shown in FIG. 15A, monitoring cameras 901 each of which includes a wireless relay apparatus, as a specific example. The monitoring cameras 901 are fixed and are not mobile. Another specific example is shown in FIG. 15B. The network therein includes movie cameras 902 (mobile phones with cameras) each of which includes the wireless relay apparatus. The movie camera 902 is carried by the user and is mobile.
The wireless relay apparatus includes a buffer which temporarily accumulates the data received from another wireless relay apparatus which is a transmission source until the data is transferred to another wireless relay apparatus which is a transfer destination. The wireless relay apparatus includes the buffer in order to prevent the variation in the quality of wireless transmission with the other wireless relay apparatus which is the transmission source and with the wireless relay apparatus which is the destination. For example, when the effective bandwidth temporarily decreases due to the degradation in the quality of wireless transmission between the wireless relay apparatus and the other wireless relay apparatus which is the transmission source, the buffer suppresses the influence of the decrease in the effective bandwidth.
However, the buffer overflows when the reduction in the effective bandwidth exceeds the capacity of the buffer. As a result, the packets to be transmitted are disposed, and if the data to be transmitted is video data, the video becomes blurry and the reproduction delays.
In other words, in the ad-hoc network, the wireless relay apparatuses are wirelessly connected. This is likely to cause buffer overflow in the wireless relay apparatuses due to the effect of the variations in the radio band, that is, the communication failure is likely to occur. The communication failure refers to a failure in communication caused by errors in transmission and by congestion. The communication failure assumes, for example, a case where the congestion occurs in the wireless relay apparatus due to the traffic interference as a result of converging traffic and a variation in the physical bandwidth, and a case where the radio wave interference due to the obstacles and movement of the wireless relay apparatus itself cause the errors in transmission.
Accordingly, in the ad-hoc network, various approaches have been proposed as a method to achieve a low-delay, high-quality transmission without packet loss.
FIG. 16 shows the congestion control by the wireless relay apparatuses.
For example, as shown in FIG. 16, the transmission terminal 903 wirelessly transmits the data to the receiving terminal 904 via the wireless relay apparatuses 900. Here, each wireless relay apparatus 900 includes a buffer bf, and transmits and receives the data via the buffer bf. In this case, the wireless relay apparatus 900 appropriately controls the rate at the application level and the timing of transmission in the Media Access Control (MAC) layer, and thereby adjusting the amount of data transmitted from the transmission terminal 903 and other wireless relay apparatuses 900 to prevent the buffer from overflowing, avoiding the congestion. Therefore, it is possible to achieve high-quality transmission.
However, the congestion cannot be solved merely by controlling the amount of transmission, because the congestion also occurs due to the radio wave interference caused by the radio control regarding the antenna such as the radio wave frequency and directivity and the magnitude of the radio wave output from the wireless relay apparatus.
FIGS. 17A and 17B are explanatory diagrams for describing the congestion caused by the radio wave interference.
For example, as shown in FIG. 17A, the congestion occurs due to the radio wave interference between the wireless relay apparatuses 900a to 900d relaying the parallel flows of traffic. For example, when the wireless relay apparatus 900a receives the data and transmits the data to the wireless relay apparatus 900c, the wireless relay apparatus 900a receives, not only the radio wave for the data, but also the radio wave output from the wireless relay apparatus 900b. As a result, the congestion caused by the radio wave interference occurs in the wireless relay apparatus 900a. 
Furthermore, for example, as shown in FIG. 17B, the congestion occurs due to the radio wave interference among the wireless relay apparatuses 900a to 900e relaying the intersecting traffic. For example, the wireless relay apparatus 900e receives the data from the wireless relay apparatus 900a and receives data from the wireless relay apparatus 900c as well.
As a result, the congestion caused by the radio wave interference occurs in the wireless relay apparatus 900e. In the cases shown in FIGS. 17A and 17B, the radio wave attenuates when the radio waves in the traffic flows interrupt each other, causing the congestion.
In order to solve this problem, a wireless relay method for avoiding the congestion caused by the radio wave interference has been proposed. In the wireless relay method, coordination with the Quality of Service (QoS) has been established, and the signal transmission output is controlled or the channel is changed (for example, see Patent Literature 1). According to the wireless relay method of Patent Reference 1, the radio control on the congested sections is performed by adjusting the signal transmission electricity, the pattern, and/or gain, based on the determination that the QoS metrics of the selected route falls below the QoS threshold.
Furthermore, a wireless mesh network where each node performs beam forming (control the directivity of the radio wave) has been proposed as well (for example, see Non-patent Reference 1). More specifically, according to the wireless relay method in the wireless mesh network in Non-Patent Literature 1, after the communication route for distributing the load is set, a new communication route for the link that could cause interference between different routes is set such that the interference is reduced, and the direction of the beam of the antenna is controlled based on the new communication route.